Differential pressure gauge for cryogenic fluids which selects a density value based on pressure measurement

ABSTRACT

A differential pressure gauge for a cryogenic storage tank provides onboard entry, by an operator, of tank dimensions, tank orientation stratification coefficient, and the type of liquid stored within the tank. A differential pressure sensor supplies a signal corresponding to a differential pressure. A pressure sensor supplies a signal corresponding to the head pressure. The gauge uses the information supplied by an operator, combined with stored formulas and liquid characteristics, to perform real-time liquid volume computations. The liquid volume may be displayed on the gauge itself or may be transmitted via telemetry to an external device.

RELATED APPLICATION

[0001] This application is a continuation-in-part of commonly assignedU.S. patent application Ser. No. 09/628,621 entitled DIFFERENTIALPRESSURE GAUGE FOR CRYOGENIC FLUIDS filed Jul. 31, 2000.

TECHNICAL FIELD

[0002] The invention relates generally to the storage of cryogenicfluids and, more specifically, to an improved differential pressuregauge that performs real-time calculations of liquid volumes so that thegauge is easier to use and can be configured to various models ofcryogenic containers.

BACKGROUND OF THE INVENTION

[0003] Cryogenic liquids, such as nitrogen, argon, nitrous oxide,oxygen, carbon dioxide, hydrogen, and the like, liquify at extremelycold temperatures. Unique problems are encountered in handling andstoring cryogenic liquids because the liquids undergo density changes atvarious storage pressures. A cryogenic storage system contains aninsulated tank for containing a cryogenic liquid in a liquid space. Eventhough the tank is insulated, heat will enter the tank, causing theliquid cryogen to slowly vaporize to a gas and, as a result, causing thevolume of liquid in the tank to diminish. This vaporization creates apressurized head space in an upper portion of the tank.

[0004] Differential pressure gauges and sensors are well known in theart for aiding in monitoring the volumes of liquids. A differentialpressure sensor senses the difference between a pressure at the headspace of the tank, or head pressure, and a pressure at the liquid spaceof the tank, or liquid pressure, also known as column pressure. Theliquid pressure is affected by both the pressure created by the headspace of the tank and the pressure due to the weight of the liquid inthe liquid space above the liquid space measuring point. By measuringthe pressure difference between the pressure at the liquid space and thepressure at the head space, the differential pressure sensor senses thepressure solely attributable to the weight of the liquid. Typically,this pressure is measured either in pounds per square inch (psi), or ininches of water column.

[0005] By dividing the sensed differential pressure by the density ofthe liquid, the height of the liquid above the liquid space measuringpoint may be calculated. This liquid height can then be displayed on thegauge. Determining the volume of the liquid in the container is moredifficult, however. Once the differential pressure has been measured, anoperator must turn to a calibration chart, separate from the gauge, todetermine the liquid volume. Calibration charts are also required inorder to determine a total liquid weight, or an equivalent gas volume(typically measured in standard cubic feet). The relation between thedifferential pressure measured by the sensor and the liquid volume ofthe tank is affected by the tank shape, dimensions, and orientation, aswell as the liquid density. Each calibration chart is therefore uniquelydesigned for a particular cryogenic tank model, tank orientation, typeof cryogenic liquid and expected pressure of the liquid. The liquiddensity is a function of the liquid type and the state of its pressure.In order to determine a liquid volume level, the operator must procurean appropriate chart and use the differential pressure reading with thechart. Such calibration charts are awkward to use, and separate chartsare required for different combinations of the factors listed above.This prevents efficient on-site monitoring of the liquid volume.

[0006] There is a need in the art for a method of determining, inreal-time, a liquid volume using an on-site differential pressure gauge.

[0007] There is a further need in the art for a differential pressuregauge that does not require the use of calibration charts in order todetermine a liquid volume.

SUMMARY OF THE INVENTION

[0008] These needs and others are met by an improved differentialpressure gauge, which allows real-time calculations of liquid volumesbased upon the reading of a differential pressure sensor, an absolutepressure sensor, and upon initial, one-time inputs by an operator. Theseinputs do not require the use of a calibration chart. The gauge can beconfigured to work with most cryogenic storage tanks. The gauge receivesdata from a differential pressure sensor, which senses the pressuredifference between the head space and the liquid space of a cryogenicstorage tank. The gauge includes a keypad, a micro-controller, and adata display for level and pressure. The gauge further receives datafrom a pressure sensor, which senses the pressure in the head space.

[0009] In operation, a user initially inputs programming informationinto the gauge, such as the dimensions of the tank, the orientation ofthe tank, the desired units of display, type of liquid, stratificationconstant and any zeroing out calibration values (not to be confused withan entry based upon a calibration chart). Once the user has input thenecessary programming information, the input data is stored, preferablyin a nonvolatile memory such as an EEPROM, and it is not necessary toinput the information again. Only if the information needs to be changed(as would be required by replacing the tank or the type of liquifiedgas) is further user action required. The gauge contains storedinformation such as cylinder dimensional formulas, unit conversionformulas, and properties (such as liquid density) of the liquified gasspecified by the user. The formulas and properties are stored in memorycontained on the onboard computer.

[0010] To determine a liquid volume present at a particular instant, thedifferential pressure sensor sends an analog signal, corresponding to adifferential pressure, to the onboard computer contained within thegauge. The analog signal is converted to a digital signal. The gaugeanalyzes this digital signal along with the initial input informationfrom the user, and a signal from a pressure sensor using stored formulasand properties to calculate a liquid volume. The results are displayedon the device, or may be transmitted via telemetry to a remote device ofthe user's choosing. Because a visit by a human operator to a site isnow not needed just to ascertain liquid volume, site visits by supplytrucks can be minimized and can be automatically triggered by the gaugedetecting that a tank's liquid volume has fallen below a predeterminedlevel.

[0011] The following detailed description of embodiments of theinvention, taken in conjunction with the appended claims andaccompanying drawings, wherein like characters represent like parts,provide a more complete understanding of the nature and scope of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1A is an elevational view of a typical cryogenic storagevessel fitted with the differential pressure gauge of the presentinvention;

[0013]FIG. 1B is a block diagram of the cryogenic storage vessel of FIG.1 fitted with the differential pressure gauge of the present invention;

[0014]FIG. 1C is a sample table containing conversion coefficients usedto calculate an average pressure;

[0015]FIG. 1D is a sample table containing liquid densitiescorresponding to various combinations of liquid type and pressureranges;

[0016]FIG. 2 is an exploded view of a differential pressure gauge andsensor according to the present invention;

[0017]FIG. 3 is a bottom perspective view of a differential pressuresensor according to the present invention;

[0018]FIG. 4 is a top perspective view of an inside face of adifferential pressure sensor housing portion;

[0019]FIG. 5 is a cross-sectional view of a differential pressure sensorhousing portion;

[0020]FIG. 6 is a differential pressure chip, in an embodiment of thepresent invention;

[0021]FIG. 7 is a block diagram for liquid volume calculations for thedifferential pressure gauge of the present invention;

[0022]FIG. 8 is an elevational view of a horizontally-oriented cryogenicstorage vessel;

[0023]FIG. 9A is an illustration of a keypad for an embodiment of thedifferential pressure gauge of the present invention;

[0024]FIG. 9B is an alternate view of the keypad shown in FIG. 9;

[0025] FIGS. 10A-10B are a flow chart showing a process for operation ofthe differential pressure gauge; and

[0026]FIG. 11 is a flow chart showing a process for inputting parametersfor the differential pressure gauge.

DETAILED DESCRIPTION OF THE INVENTION

[0027]FIG. 1 shows a cryogenic storage vessel fitted with thedifferential pressure gauge of the present invention, the vessel beingindicated generally at 10. Vessel 10 contains an inner tank 12 forholding cryogenic liquid. The tank 12 comprises a shell portion 20 andbottom and top convex end portions 21, 22. The top of the liquid levelis shown at 24, and the height of the inner liquid is indicated by hi.The tank has overall tank length l_(t) and inner diameter d_(i). Anouter jacket 14 surrounds inner tank 12, forming an insulation chamber16 between the jacket 14 and tank 12. The insulation chamber 16 isfilled with an insulation material (not shown), and a vacuum is createdwithin chamber 16 to minimize the heat transfer between the externalenvironment and the interior of the tank 12.

[0028] As heat from outside of the vessel 10 transfers into the tank, aportion of the liquid at the bottom of the container will vaporize andmove to the top of the container, separating the contents of the tankinto a liquid space 26, and a head space 28. The head space pressure canbe significant as pressure builds within tank 12. The pressure at thebottom of the liquid space includes both the pressure of the head space28 and the weight of the liquid in the liquid space 26. Tap lines 32,34provide communication between both the liquid space and the head spaceand a differential pressure sensor 30, via 4-way valve 36. The tap linesare preferably tubing formed of copper or other metal. Tap lines 32, 34are fed into two connections 312, 314 (shown in FIG. 3) of thedifferential pressure sensor 30 and fitted to the sensor 30, preferablyusing elbow brass fittings. The differential pressure sensor 30 is inturn connected to a differential pressure gauge 40 (see FIG. 2) forcalculating, displaying, and transmitting information about thedifferential pressure and liquid volume of the cryogenic fluid withinthe cylinder 20. The information may also be transmitted to a personalcomputer (not shown) via any suitable means, including but not limitedto modem, infrared, LAN, IR, serial port, USB, and wireless means.

[0029]FIG. 2 is an exploded view of the differential pressure gauge 40and differential pressure sensor 30 of the present invention.Differential pressure gauge 40 includes an operator input interface inthe form of a membrane keypad 44, shown in more detail in FIG. 9, forconfiguring the gauge. The keypad 44 is electrically connected to anonboard computer 46, containing a processor such as a micro-controlleror a microprocessor, the keypad being physically separated from theonboard computer by a cover 48. An onboard computer 46 used in apreferred embodiment of the invention is model number MC68HC711E20CFN2manufactured by Motorola, Inc. of Schaumburg, Ill. The onboard computeris adapted to calculate the liquid volume. The onboard computer 46 andcover 48 are housed within enclosure 50, which is waterproof.

[0030] Sensor 30 is connected to differential pressure gauge 40 by setscrews 49, and preferably separated from the gauge by a gasket 54. Thesensor 30 in a preferred embodiment includes a housing 300, preferablyof brass, for containing a differential pressure sensor chip 320. FIGS.3-5 show sensor 30 in more detail. To manufacture the sensor 30, theplastic-enclosed chip 320 is placed between first and second housingportions 302, 304 (FIG. 3). Housing portion 302, an inside surface ofwhich is shown in perspective in FIG. 4, and shown in cross-section inFIG. 5, is tap drilled so that a first surface of chip 320 (containing apassageway to the central transducer) is exposed through a tapconnection 312 extending from the inside surface through the firsthousing portion 302 to the outside surface. One such chip 320 is shownin FIG. 6. A differential pressure sensor chip in a preferred embodimentis a plastic-enclosed Motorola MPX5050 series piezoresistive transducer,manufactured by Motorola, Inc. of Schaumburg, Ill. Alternative sensorsmay be used providing that the disclosed functionality is implemented.The brass housing 300 allows the plastic-enclosed chip to be used forthe present invention, by helping the chip withstand the large absolutepressures that are transferred to the chip.

[0031] A second housing portion 304, being manufactured similarly toportion 302, is mated to first housing portion 302 over chip 320 so thatopposing surfaces of the chip are exposed through tap connections 312through both housing portions 302, 304. In this way, tap lines 32 and 34(FIG. 1) communicate with each side, respectively, of the chip 320within its plastic enclosure.

[0032] A pair of O-rings 308 (FIG. 5) between opposing surfaces of thechip 320 and the inside surface of housing portions 302, 304 ensure atight fitting of the chip 320, sealing the chip within the housing 300.Each O-ring 308 is disposed within an annular groove 330 for maintainingthe position of the O-rings 308 within the housing portions. The annulargrooves 330 extend inwardly from channels 326 (shown in FIG. 4) towardthe outside face of each housing portion. The grooves are concentricwith and surround the inside face of tap connections 312, as shown inFIG. 5. The two housing portions are bolted together around the chip 320and O-rings 308 until the halves bottom out. The housing portions, whenassembled may be separated by a rubber gasket (not shown) having acentral opening. The assembled housing portions exert significantpressure on the O-rings 308, and thus onto the outside of the chip 320,pre-loading the chip. The pre-loading pressure is created by anapproximately 30% (±3%) compression of O-rings 308 against both sides ofchip 320. By securing the plastic-enclosed sensor chip 320 within thebrass housing 300, the forces applied to the outside of the chip holdthe plastic-enclosed chip together against the significant absolutepressures exposed to both surfaces of the chip via tap lines 32, 34(FIG. 1). A gas-tight seal is thus formed about the chip even at highpressure. The housing strengthens the chip, thus preventing the chip,and case, from exploding under the significant absolute pressureintroduced. The plastic-enclosed chip has a maximum operating absolutepressure of 100 psi, but when encased in the brass housing, it has amaximum operating absolute pressure of at least 750 psi. The cost ofthis preferred differential pressure sensor is significantly lower thanprior sensors, because an off-the-shelf chip, with low absolute pressuretolerance, is adapted to withstand large absolute pressures whilemeasuring differential pressure. This is much more cost-effective thanmanufacturing a customized diaphragm within a housing.

[0033] Channel 326 extends inwardly from the inside face of each housingportion. The channel contains a circular segment surrounding the insideface of tap connection 312 and adapted to contain chip 320 when combinedwith the other block. Channel 326 has a longitudinal segment extendingfrom the circular segment along the inside surface of the housingportion towards an outer edge of the housing portion. When facingchannels 326 from housing portions 302, 304 are combined (as the housingis assembled) they form a passageway 328 for housing the chip, and forelectrical leads 322 running from the transducer of the sensor chip 320to the differential pressure gauge 40.

[0034] Turning to FIGS. 1A and 1B, (gauge) pressure sensor 31 isprovided for measuring the head space pressure within tank 12. Tap line34 provides communication between the head space and pressure sensor 31,via valve 36. Tap line 34 is preferably tubing formed of copper or othermetal. The pressure sensor 31 is in turn connected to the onboardcomputer 46.

[0035] The onboard computer 46 reads a conversion coefficient SC fromlookup table 84 (FIGS. 1B, 1C, 7) according to the shape and orientationof the cylinder 20. Next, the onboard computer 46 calculates anestimated average pressure (P1) by subtracting the predeterminedconversion coefficient (SC) from the head pressure (P) read from thepressure sensor 31. In turn, the density of the cryogenic fluid is readfrom a lookup table 84 (FIGS. 1B, 1D and 7), using the adjusted pressureP1 and the liquid type.

[0036] Preferably, the lookup table 84 is stored in memory 85, which isa nonvolatile memory such as a ROM, EEPROM or the like.

[0037] The conversion coefficient SC is empirically determined, andvaries depending on the size and orientation of the cylinder 20.

[0038]FIG. 7 is a block diagram of the operation of liquid volumedetermination for the sensor and gauge of the present invention. Thisdiagram shows the inputs and outputs of the differential pressure gaugewhen calculating a liquid volume. The onboard computer 46 receivesinitial operator input data 62 via an operator input interface, such askeypad 44. This input data may include physical data 64 for thecontainer, including the container's diameter and height, and theaforementioned stratification coefficient SC 73. The desired unit ofdisplay 66, type of liquified gas 68, calibration offset values 70, andalert levels 72 are similarly input by the operator and received by theonboard computer 46 where they are stored, preferably in EEPROM. Oncethis input data is entered into the gauge by a user, the user does notneed to reenter the information. Only if the user wishes to change theconfiguration input parameter (units, for example), does the user needto enter additional information. The gauge applies the input data todetermine liquid volumes, without further input required from the user.

[0039] Dimensional formulas for the container 80, liquefied gasconstants 82 such as density, and unit of measure conversion formulas 84are stored in memory 85, such as a programmable ROM or EEPROM, for usein calculating the liquid volume in the selected unit. In alternativeembodiments, certain elements of the operator input data, such ascylinder physical data 64 and desired unit of display 66, could becalibrated at the factory and stored in memory 85 (such as a PROM orEEPROM) so that the operator of the unit would not need to enter them inthe field.

[0040] In operation, differential pressure sensor 30 sends an analogsignal, dependent upon the detected differential pressure, on path 47(corresponding to leads 322 (in FIG. 6)) to an analog-to-digitalconverter 74, which in turn sends a digital signal on path 75 to themicroprocessor 76 for processing. Similarly, pressure sensor 31 sends ananalog signal, dependent on the head pressure detected on path 51 to ananalog-to-digital converter 74, which in turn sends a digital signal onpath 75 to the microprocessor 76 for processing. In calculating liquidvolumes, the microprocessor periodically interprets the digital signalon path 75 to determine the differential pressure. From this signal, theliquid volume is calculated in real-time, in a preferred embodiment, asfollows.

[0041] The digital signal received by the gauge from path 75 isrepresented by a raw value of counts. With the preferred sensor chipused (3.0 V DC), the counts are related to differential pressure by aratio of 255 counts per 3 psi. Alternative ratios are possible,particularly if different sensors are used. By dividing the raw value ofcounts by this stored ratio, a differential pressure is calculated.

[0042] Similarly a signal is received from the pressure sensor. Thissignal corresponds to the pressure in the tank and calculates saturationpressure using the SC constant. The gauge then retrieves from table 84(FIG. 10) one of a set of density constants to calculate liquid heighth_(i).

[0043] To reduce the number of entries in the look-up table 84, thepressure value P1 may be rounded, for example to the nearest 10 PSIvalue.

[0044] The differential pressure is divided by the liquid density toobtain a liquid height h_(i), in inches. The tank length l_(t) anddiameter d, input by the user, and the liquid height h_(i), are thenused to calculate the liquid volume, for either vertical or horizontaltanks, according to the respective formulas given below.

[0045] For vertical tanks, the gauge first calculates the length ofshell 20, shown as l_(s) in FIG. 1, by assuming that the bottom and topconvex ends 21, 22 are 2:1 oblately ellipsoidal shapes and that eachhave a height equal to (d/4), therefore

l _(s) =l _(t)−2*(d/4).

[0046] The gross (total) filled tank volume V_(tank) is calculatedusing:

V _(tank)=0.93((π/4)d ²(l _(s))+π(d ³/12))

[0047] with constant π stored in memory to ten significant digits, andconstant 0.93 used to adjust for normal fill level of the tank.

[0048] Then, the volume of liquid V₁, in cubic inches, is calculatedusing one of four formulas, depending on whether the liquid level 24 iswithin bottom end 21, shell 20, top end 22, or fills the tank,respectively.

[0049] If h_(i)<d/4:

V ₁ =h _(i) ²(πd−(4/3)πh _(i))

[0050] If h_(i)<l_(s)+d/4:

V ₁=(π/24)d ³+((π/4)d ²(h _(i) −d/4))

[0051] If h_(i)<l_(s)+d/2:

V ₁=(π/24)d ³+(π/4)d ² l _(s)+(π/4d ²(h _(i) −l _(s) −d/4)−(4/3)π(h _(i)−l _(s) −d/4)³)

[0052] else

V₁=V_(tank)

[0053] Alternatively, liquid volume calculations may be performed forhorizontally-oriented vessels, such as vessel 10 a, shown in FIG. 8. Thevessel contains horizontally-oriented tank 12 a, outer jacket 14 a,insulation chamber 16 a, and an interior volume divided between liquidspace 26 a, and head space 28 a. Tank 12 a is composed of shell 20 a andleft and right convex end portions 21 a, 22 a. The liquid level is shownat 24 a. For this tank, having liquid height hi, inner tank diameter d,and tank length l_(t), the volume is calculated by adding the liquidvolume in the left and right ends 21 a, 22 a, each assumed to have alength of (d/4), to the liquid volume in shell 20 a. The liquid volumeV₁, in cubic inches, is thus calculated using:

V ₁ =h _(i) ²((π/4)d−(π/6)h _(i))+(({square root}h _(i)(d−h_(i))(h−d/2)+(d ²/4)*sin⁻¹((2/d)(h _(i) −d/2)))*l _(s)

[0054] where l_(s)=l_(t)−2*(d/4)

[0055] These volume calculation formulas are exemplary only. It shouldbe apparent to those skilled in the art that alternative formulas may beprogrammed and contained within the memory of the differential pressuregauge for performing volume calculations. Alternatively, the computer 46may be preprogrammed with lookup tables which, given the height, willreturn approximately the same results as the above algebraic formulas.In yet another embodiment, these tables may be produced via calculationafter initial input and the results stored within memory, with theruntime operation then limited to a fast table lookup.

[0056] By dividing V₁ by V_(tank) and multiplying by 100%, a fillpercentage can be calculated. This fill percentage may be adjustedaccording to an entered calibration value. The fill percentage isrounded to the nearest 5% for display.

[0057] As opposed to the prior art, the liquid volume calculations canbe performed in real-time, without the user resorting to calibrationcharts that depend upon particular cylinder models, cylinderorientations, and liquid types. The user, after initial programming, isnot required to input or analyze data before receiving a liquid volumeresult, and is not required to monitor the gauge. As shown in FIG. 7,the calculated liquid volumes are converted to display values at 78 andare displayed at 86 on the gauge.

[0058] The results may also be transmitted via telemetry for remote orautomated monitoring of liquid volume. In a preferred embodiment, gauge40 is coupled to a phone transmitter 43 (FIG. 1) for transmitting tankstatus information, such as liquid level, liquid volume, equivalent gasvolume, pressure, or fill percentage, to remote devices 45. The phonetransmitter 43 also may send an alert to an operator if the liquidvolume within tank 12 falls below a particular level. The telemetrysystem may also schedule delivery of additional liquid cryogen, asneeded. Phone transmitter 43 is coupled to a phone line, or other signaltransmitting line. In addition to, or alternative to the phonetransmitter 43, gauge 40 may be coupled to a satellite transmitter or acellular transmitter (not shown).

[0059] The gauge 40 may be preprogrammed to send a signal to phonetransmitter 43 indicating that an alarm has been tripped. Whentransmitter 43 receives this signal, it relays the information on thestate of the tank (such as the liquid volume) to a remote device 45,such as a PC, via the telephone line. Of course, alternative means oftransmission are possible. The remote device then signals that an alarmhas been received and needs attention. For example, an audible alarm maybe used. The remote device may be programmed to automatically notify adriver of a liquid cryogen delivery vehicle of the alarm andcorresponding liquid volume, via pager, email, or other methods.

[0060] In normal (non-alert) operation, within these remote devices, thecalculation results, as well as raw differential pressure sensorreadings, may be stored for aggregation, reporting, graphing, or otherpurposes. By use of these telemetry features, an operator, after aone-time input of tank parameters into the gauge 40, can remotelymonitor the storage container 10 only when desired, with the gaugeitself managing the amount of liquid within the inner tank 12. Also,fewer visits by a delivery vehicle are needed, because the gauge may beprogrammed to notify the vehicles for resupply only if the tank 12volume is below a desired amount.

[0061] Before the differential gauge can calculate liquid volumes, anoperator must initially input data parameters. These parameters may beentered by an operator, using display keypad 44, or via telemetry, asdescribed above. FIG. 9A shows an embodiment of a keypad for thedifferential pressure gauge of the present invention. An alternate viewof the keypad is shown in FIG. 9B. A clear display portion 400 allowsnumerical LED's contained on computer 46 to be visible to the operator.Other LED's visible on the keypad indicate settings and alerts. TheLED's corresponding to the units chosen by the operator are shown at402. The alert LED's are shown at 404. Three keys 406, 408, 410, shownat the bottom of the sample display, allow the user to configure thegauge. When programming the gauge, the increment key 406 eitherincreases the displayed reading by an increment that depends on the unitbeing programmed, or it scrolls through a list of options within afield. The select key 408 allows the user to accept a displayed resultwhen programming the gauge, and may also move the gauge to a next set ofparameters. To enter a number, for example, ‘345’, an operator pushesthe select key 408 until the hundreds digit is blinking on the display.The operator scrolls through the digits in the hundreds position bypushing the increment key 406 until a ‘3’ is reached. The operator thenpushes the select key 408, causing the tens digit to blink. The operatorsimilarly increments to and selects ‘4’ in the tens position, and then‘5’ in the ones position. After pressing the select key 408 at thisfinal position, the chosen value or code is stored in EEPROM so that itdoes not need to be reentered as the gauge calculates liquid volumes.The “ON” key 410 allows an operator to start the gauge. Holding the “ON”key for a period of time puts the gauge in diagnostic mode.

[0062] FIGS. 10A-10B show a flow chart of a process for operation of thegauge of the present invention. After powering up, the gauge performsappropriate diagnostics at 100, checking gauge functions such as, butnot exclusively, the functions of the microprocessor, EEPROM, alertswitches, serial ports, or the included software. A check of thepower-up at 102 follows. If one or more of the gauge functions arefaulty, an appropriate error code may be set at 103. If the diagnosticsare successful, the gauge reads the differential pressure at 104. Thegauge performs a second diagnostic check at 106 based at least partlyupon the differential pressure read. If the diagnostic fails for reasonssuch as an inappropriate pressure range, a faulty alert output, or aninappropriate calibration status, an error code is set at 108. If thesettings are okay at 110, the gauge will begin taking pressure readingsand calculating liquid volumes at 120. If not, the gauge goes to anappropriate menu at 112.

[0063] When taking readings at 120, the gauge reads a differentialpressure at 122, and determines the display mode at 124 (FIG. 10A) thata user selected based upon previous inputs. These modes include, but arenot limited to: percent full (% full), liters, pounds, kilograms, psi,inches of water column (pressure), inches (height), standard cubic feet,or normal cubic meters.

[0064] The computer reads the pressure at 125A. The computer acquires,at 126, the necessary formulas, dimensions and liquid properties, andperforms the appropriate calculations or their equivalents at 128 asdescribed above, though, again, many of these calculations could bereduced to lookup tables stored within EEPROM prior to runtime.

[0065] In one embodiment, the gauge performs a volume calculation in astandard unit (such as cubic inches) and then, based upon the selectedunit of display, converts the volume to the desired unit, using storedunit conversion formulas and/or constants, shown at 84. If a liquidheight or differential pressure is chosen, the gauge makes theappropriate calculations described above, for these units as well. TheLED corresponding to the selected mode is lighted at 130. LED'scorresponding to the modes not selected are turned off at 132. Therelated readout is displayed at 134. For percent full, for example, asample reading could be “50%.” In a preferred embodiment, the displayedfill percentage is a multiple of 5%, but this percentage may be in anyincrement. If the pressure sensed by the differential pressure sensor isless than a minimum amount desired at 136, as set by an operator, thegauge can indicate this by, for example, an alert message will be shownon display 400. Otherwise, the alert LED is turned off at 138. More thanone alert level may be programmed into the device and analyzed. If thebattery voltage is below a particular desired level at 142, an alertmessage may be shown on display 400 (FIG. 10C). Otherwise, the LED isturned off 146. The gauge determines if the user wishes to go toparameter input mode at 148, or diagnostic mode at 149. The userindicates this by holding one key or a combination of keys for aparticular period of time. If the user does not indicate an alternatemode, the gauge again begins to take readings 120 (FIG. 10A). As long asthe gauge is powered and not interrupted by the user, the gauge willcontinue to take periodic readings and display the results, withoutfurther input required from the user.

[0066] By pressing and holding the select and increment keys for acertain period of time, the operator can begin a parameter input mode at200, shown in FIG. 1. Before accepting new inputs, the gauge may clearthe parameters stored in the EEPROM. The operator, by scrolling with theincrement key and selecting with the select key, enters parameters,starting with the liquid type at 202. The liquid type may include liquidnitrogen, argon, nitrous oxide, oxygen, carbon dioxide, or others. Theoperator enters a tank length at 204, and a tank diameter at 206. Thetank length and diameter may be configured by a similar system ofselecting and incrementing digits as described in the alarm settings.The tank orientation is entered by the user at 208. The increment key isused to rotate the tank orientation between a vertical orientation and ahorizontal orientation.

[0067] Also in parameter input mode 200, it is desired that an operatorenter a desired stratification coefficient. This entry accommodates forvariations in the liquid stratification as the height of the liquidincreases. The gauge uses the exact liquid pressure for determining aproper liquid density in volume calculations by comparing the liquidpressure to a set of liquified gas properties, including liquid densityvalues, stored in nonvolatile memory, as described above. The gauge mayalso accept a calibration, or zeroing out, parameter at 212. Thisparameter allows an operator to calibrate the “% Full” reading, which isthe percentage of volume within the tank which is filled with liquid. Ina preferred embodiment, the zeroing out parameter ranges from −9 to +9,in increments of one, with each increment representing a 3% calibrationin the displayed fill percentage. At value “zero”, there is nocalibration. With this method, the operator may program the gaugewithout resorting to a chart to determine the proper inputs. Theoperator may then program one or more alert levels at 220, so that analert will be indicated if the pressure or volume in the tank fallsbelow a certain level. For example, by holding the increment key andselect key, an operator can configure a first alert level. Once in themode of configuring the first alert level, the user enters the alertlevel in hundreds, tens and ones, in the manner previously described.The programming may be a step in program mode, or alternatively, anoperator may directly enter the alert setting mode at 220 by, forexample, holding one or more keys for a particular length of time. Oncethe programming is complete, and the values are stored at 221, the gaugereturns at 222 to reading mode (step 120, FIG. 10A) to take readings,perform real-time calculations, and display results.

[0068] While the preferred embodiments of the invention have been shownand described, it will be apparent to those skilled in the art thatchanges and modifications may be made therein without departing from thespirit of the invention, the scope of which is defined by the appendedclaims.

We claim:
 1. A method of determining in a volume of liquified gas in acryogenic storage tank having a liquid space and a head space,comprising the steps of: entering and storing the dimensions andorientation of the storage tank; entering and storing the type ofliquefied gas contained in said tank; entering and storing astratification coefficient; storing a lookup table of conversioncoefficients used to covert a head pressure value into an estimatedaverage pressure value, said conversion coefficients being accessedaccording to the entered dimensions and orientation of the storage tank;storing a lookup table of density constants, stored according to theestimated average pressure, and liquid type; measuring the differentialpressure between the liquid space and the head space; measuring thepressure at the head space; calculating an estimated average pressureusing the measured head pressure and the stratification coefficient;reading a liquid density value from a look-up table of density constantsusing the calculated estimated average pressure and the entered liquidtype; and computing in a liquid volume in said tank as a function ofsaid differential pressure, liquid density tank dimensions, tankorientation, and type of liquefied gas.
 2. The method of claim 1,wherein the step of computing the liquid volume further comprises thestep of displaying the liquid volume on a display.
 3. The method ofclaim 1, wherein the step of storing the dimensions of the storage tankcomprises the step of storing a tank height and diameter.
 4. The methodof claim 1, further comprising the steps of: storing an alert volume;and generating an alert signal if the calculated liquid volume is belowthe alert volume.
 5. A system for displaying liquid volume of acryogenic fluid stored in a cryogenic tank, the fluid stored within thetank, the tank containing a liquid space and a head space, the systemcomprising: a differential pressure sensor coupled to the head space andto the liquid space for sensing a differential pressure between theliquid space and head space and generating a differential pressuresignal as a function of the differential pressure; a pressure sensorcoupled to the head space for sensing a pressure of the head space; anoperator input interface for entering the dimensions of the containerand the type of cryogenic fluid; a nonvolatile memory coupled to theoperator input interface, for storing the dimensions of the tank, thetype of cryogenic fluid, tank dimension formulas, a look-up tablecontaining liquid density conversion constants for each of plural tankdimensions, and a look-up table containing cryogenic fluid densities foreach of plural liquid types and plural pressure ranges; amicro-controller coupled to the pressure sensor, the differentialpressure sensor and the nonvolatile memory, said micro-controllerreading from said nonvolatile memory a selected liquid densityconversion constant corresponding to said stored dimensions of thecontainer; said micro-controller calculating an estimated averagepressure as a difference between said head space pressure and saidliquid density conversion constant, and reading from said nonvolatilememory a selected liquid density corresponding to said estimated averagepressure and said liquid type; said micro-controller calculating aliquid volume based upon the dimensions of the tank, the type ofcryogenic fluid, the tank dimension formulas, and the cryogenic fluiddensities; and a display coupled to the micro-controller for displayingthe liquid volume.